Customizer: Fix moving focus to available nav menu items search.

Fixes regression introduced in [34219].

Cherry-picks [34829].

Props tyxla.
See #33360.
Fixes #34125 for 4.3.

Built from https://develop.svn.wordpress.org/branches/4.3@34830


git-svn-id: http://core.svn.wordpress.org/branches/4.3@34795 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/css/customize-nav-menus-rtl.css

@@ -609,8 +609,8 @@
 	border-right: none;
 	border-left: none;
 	background: #fff;
-	-webkit-transition: all 0.15s;
-	transition: all 0.15s;
+	-webkit-transition: background-color 0.15s;
+	transition: background-color 0.15s;
 }
 
 #available-menu-items .open .accordion-section-title,

wp-admin/css/customize-nav-menus.css

@@ -609,8 +609,8 @@
 	border-left: none;
 	border-right: none;
 	background: #fff;
-	-webkit-transition: all 0.15s;
-	transition: all 0.15s;
+	-webkit-transition: background-color 0.15s;
+	transition: background-color 0.15s;
 }
 
 #available-menu-items .open .accordion-section-title,